Resins for extrusion coating on paper, board, aluminum, etc., are designed with broad molecular weight distribution and low extractables. In extrusion coating applications, the polymer is processed at high temperature conditions, typically above 280° C. and below 350° C. Broad molecular weight distribution (MWD) resins with a very high molecular weight fraction are used for good processability during coating (neck-in and drawdown balance). Low extractables are needed to reduce undesirable taste and odor, and to reduce smoke formation during the processing of the resin, especially when the resin is subjected to the high coating temperatures.
Typically LDPE (low density polyethylene) resins with broad MWD are made using autoclave reactors or a combination of autoclave and tube reactors. Broad MWD resins can be made by promoting long chain branching, and through the inherent residence time distribution, by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths.
Broad MWD autoclave resins for LDPE extrusion coatings are focused in two product density regimes, namely from 0.915 to 0.919 g/cc and from 0.919 to 0.924 g/cc. The invention in this document describes improved broad MWD tubular reactor products designed for the higher density regime from 0.919 to 0.924 g/cc.
The autoclave and tubular reactor systems differ in residence time distribution, which is more uniform for tubular reactors and dispersed for autoclave reactor zones. The uniform residence time leads to narrower MWD, and very broad MWD can only be achieved in tubular reactors by applying extremely differentiated polymerization conditions, for example, as described in WO 2013/078018, and/or application of a branching/cross-linking agent, for example, as described in U.S. Pat. No. 7,820,776. The use of extreme process conditions and/or branching/cross-linking agents can lead to high melt strength tubular low density polyethylene suitable for extrusion coating applications; however with elevated extractables. Undesirable gels in the polymer can result from the use of branching or cross-linking agents. Due to the difference in cooling capability, the conversion level ranges typically from less than (<) 20% (autoclave) to more than (>) 30% (tubular). This large difference in conversion level has a major impact on investment and operation costs as well on polymer output and power consumption (to compress ethylene) per unit of polymer.
U.S. Publication No. 2008/0242809 discloses a process for preparing an ethylene copolymer, where the polymerization takes place in a tubular reactor at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (Meth)acrylate. WO 2012/057975 describes polymers comprising monomeric chain transfer agents (mCTAs). WO 2012/084787 describes simulated tubular reactions in which bi- and/or higher functional comonomers. EP 2 681 250 B1 describes a process of preparing an ethylene polymer using a free radical initiator and at least one chain transfer agent, wherein the concentration of the CTA in the first reaction zone is less than 70% of the concentration of the CTA in the reaction zone with the highest concentration of CTA. Other polymers and processes are disclosed in WO 2007/110127; WO 2014/003837; WO 2013/078018; WO 2013/078224; WO 2013/178241; WO 2013/178242; WO 2013/149698; WO 2013/132011 and WO 2013/083285.
For a multi- and/or bifunctional component to impact polymer rheology, it is important that (1) at least two functional groups of the component molecule react, and (2) effective branches are formed in the polymer. A “C═C” type of functional group (e.g., vinyl) will act as a comonomer, and incorporate into a polymer molecule. CTA functionality will either start the formation of a new polymer molecule, or initiate, after incorporation of the monomeric group, the formation of a LCB or T-branch. When the functional groups consist of monomeric groups, H-branches can be formed. H-branches are either intermolecular (between two molecules) or intramolecular (within a molecule), and formed by reaction of two or more “C═C” type groups of the bi- and/or multifunctional component. There is a need for polyethylenes made at density from 0.9190 g/cc with broad MWD, high G′ value and at reduced extractable levels in a tubular reactor at high ethylene conversion levels. These needs have been met by the following invention.